This invention relates to an electronic speed control device for a miniature direct current motor suitable for driving light loads such as cassette or cartridge type tape recorders or record players.
Although various types of miniature direct current motors have been proposed in the past it is still impossible to obtain miniature direct current motors having small size, light weight, operating stably and inexpensive. One example of the prior art arrangement is shown in FIG. 1 and comprises a combination of a DC motor 10 and an AC tachometer generator 11. More particularly, motor 10 and generator 11 are mounted on a common shaft, not shown, so that the generator 11 produces an output proportional to the speed of the motor 10. The output from generator 11 is rectified by a diode 12. The output from diode 12 is smoothed by a capacitor 13 and the terminal voltage thereof is impressed upon the base electrode of a transistor 14. The output from transistor 14 is applied to the base electrode of a transistor 15 which is connected in series with the armature winding of motor 10. There are provided resistors 17 and 18 and a variable resistor 19 connected in parallel with capacitor 13.
The circuit shown in FIG. 1 operates as follows. When motor 10 rotates, the generator 11 produces an output voltage proportional to the speed of the motor. When the speed of the motor rises to a predetermined value, the output from generator 11 turns ON the transistor 14 thereby decreasing the collector potential thereof. Accordingly, the base potential of transistor 15 is lowered to decrease the emitter current thereof thereby decreasing the speed of motor 10. When the speed of motor 10 decreases, the output voltage of generator 11 also decreases with the result that transistors 14 and 15 operate in a manner opposite to that described above thereby increasing the motor speed. In this manner, the motor speed is regulated to a definite value. Such prior art system is disclosed in the specification of Japanese Patent No. 406479, for example.
With such construction, however, use of generator 11 increases the cost, weight and size of the motor assembly.
According to another prior art arrangement, as disclosed in the specification of United States Patent No. 2,814,012, the armature winding of a DC motor is connected in one arm of a resistance bridge circuit and the output of the bridge circuit is used to control the power supplied to the bridge circuit. With this construction, however, since the armature winding of the motor is used as one arm of the bridge circuit, the variation in the contact resistance between the brush and the commutator of the motor and the variation in the armature winding resistance due to temperature rise in the DC motor 10 being in use affect the balancing condition of the bridge circuit thereby rendering it difficult to obtain a stable constant speed. Although this problem can be solved by using noble metals for the brush and commutator, the use of noble metals increases the cost of the motor.
According to still another proposal, a brushless DC motor was used to obviate the problem caused by the variation in the contact resistance between the brush and commutator, but the circuit construction of the brushless motor is complicated and expensive so that the brushless motor is used in only high grade appliances where cost is not the problem.